1. Field of the Invention
The present invention relates to an image forming method and apparatus having a plurality of image forming units attachable to and detachable from the image forming apparatus, each including at least a developing device and respectively supplied with toners of a plurality of colors from a plurality of toner supplying devices.
2. Discussion of the Background Art
There is a background image forming apparatus including process cartridges for respective colors of yellow (Y), magenta (M), cyan (C), and black (K), each of which integrally includes at least a photoconductor and a developing device, and which is attachable to and detachable from the image forming apparatus. That is, the background image forming apparatus uses the process cartridges respectively including the developing devices containing toners of the four colors Y, M, C, and K. Therefore, manufacturers of image forming apparatuses are imposed with an extra burden, such as manufacturing management and adjustment according to colors.
In view of the above, there is another background image forming apparatus using common cartridges for toners of different colors supplied thereto. Prior to being installed in the image forming apparatus, the common cartridges are equal in structure for the toners of different colors supplied thereto, with no toner supplied in developing devices of the common cartridges. Only after being installed in the image forming apparatus and toners of the respective colors are supplied in the developing devices, the common process cartridges are differentiated from one another. In this way, the process cartridges are common in structure prior to their installing in the image forming apparatus, and thus the manufacturing management and adjustment according to colors are unnecessary. Further, such items as a seal for identifying the toner color of a process cartridge are also unnecessary. As a result, manufacturing cost can be reduced.
In another background image forming apparatus, process cartridges are differentiated from one another, with their developing devices respectively supplied with the toners of different colors prior to their installing in the background image forming apparatus. In this case, toner supplied in each developing device is precharged to a predetermined level in a production plant before shipping. For example, therefore, even after the process cartridge is left unused for a couple of months before being replaced with a new one, the toner in the developing device is kept charged to some extent. Thus, there is a relatively small difference between the predetermined toner charge level and a toner charge level at the time of replacement of the process cartridge. Therefore, if the toner in the developing device is mixed for a predetermined time period in an initial operation performed in the replacement of the process cartridge, the toner in the developing devices can be increased to a level substantially equal to the predetermined charge level, even if friction chargeability of the toner changes due to environmental or other factors.
Meanwhile, if the process cartridges are common in structure prior to their installing in the image forming apparatus, toners are supplied to the process cartridges only after the process cartridges are installed in the image forming apparatus. Therefore, the toners in the developing devices of the respective process cartridges are hardly charged, when the process cartridges are installed in the image forming apparatus and the developing devices of the respective process cartridges are supplied with the toners for the first time. In the background image forming apparatus in which the common process cartridges are installed, a toner mixing time is set to be longer than in a case in which the common process cartridges are not used, so that toner in each developing device is charged to the predetermined toner charge level. In some cases, however, even if the toner in each developing device is mixed for the longer toner mixing time, the toner in the developing device is not charged to the predetermined toner charge level. This is because of the following reason. When the developing devices of the process cartridges are supplied with the toners for the first time, there is a relatively large difference between the toner charge level of the developing device and the predetermined toner charge level. Therefore, if friction chargeability of the toner changes due to environmental or other factors, the change in the friction chargeability of the toner increases an error in the toner charge level obtained after the toner is mixed for the predetermined time period. As a result, in some cases, even if the toner is mixed for a longer time than in the case of the background image forming apparatus in which the common process cartridges are not used, the toner in each developing device is not charged to the predetermined charge level. Consequently, if the toner mixing time is set to be substantially long in consideration of influences by the environmental or other factors, the toner in the developing device may be charged to an unnecessarily high toner charge level. As a result, density of the toner may decrease, for example.